Photostriction of molecular 2D nanosheet

(Nanowerk Spotlight) Photostriction is the property of certain materials to undergo a change in internal strain – and therefore shape – with exposure to light. This means that the energy from light is converted into mechanical motion of the material.

Photostriction induced by light-matter interaction is of significant interest due to its rich photophysics and broad applications in wireless photoactuators, adaptive optics, and artificial muscle technologies.

Molecular charge-transfer crystals show promise as a novel class of photostrictive materials. In contrast to inorganic materials with high covalent or ionic bonds, organic molecular solids have weak intermolecular interactions, such as hydrogen bonds, π-π stacking, van der Waals, and charge-transfer-induced electrostatic interactions, which result in distinct lattice deformations under photoexcitation due to the large intermolecular spacing.

Furthermore, the superior optoelectronic properties of charge-transfer solids can lead to changes in the surface electric field and elastic strain due to the converse piezoelectric effect, which leads to deformation of the sample.

These clues suggest the possibility of molecular charge-transfer systems with useful photostrictive properties.

In new work employing the photostrictive effect, researchers have fabricated a flexible two-dimensional (2D) charge transfer molecular (sub-nanometer) nanosheet and observed a sizeable photostrictive effect of 5.7% with fast, sub-millisecond response; this is higher than that of some conventional ferroelectronics and polar semiconductors.

This photostrictive effect arises from excess charge carriers induced lattice dilation and conformation change, which is higher than that of some conventional ferroelectronics and polar semiconductors.

"The photostrictive mechanism presented here is very new, which extends the photostrictive research field in the research community," Shenqiang Ren, a Professor in the Department of Mechanical and Aerospace Engineering, and the Research and Education in eNergy, Environment & Water (RENEW) Institute at University at Buffalo, The State University of New York, tells Nanowerk. "In addition, our findings yield a new platform for 2D molecular charge transfer nanosheets with potential applications in flexible 2D photo-driven microsensors and actuators."

The design and fabrication of molecular charge transfer nanosheets demonstrated by a collaboration of Klein's group at Temple University, yields a new platform for 2D molecular charge transfer nanosheets with potential applications in flexible 2D photo-driven microsensors and actuators.

To fabricate their ultrathin charge-transfer network nanosheets, the team used dibenzotetrathiafulvalene (DBTTF) as the electron donor and C60 fullerene as the electron acceptor.

"The excited, cationic state of DBTTF is found to be flat, and this flattening upon photoexcitation increases the length of the unit cell, contributing to the distinct photostriction phenomenon observed in the film," explains Ren. "The fullerene C60 is known to be an acceptor possessing a number of characteristic features, namely, high-level electronic structure, lower symmetry, and intrinsic polarizability. Due to the high intermolecular interactions between DBTTF and C60 molecules, a long-range ordered packing arrangement of molecular CT nanosheets can be formed."

Due to the fast, sub-millisecond response time of the flexible 2D charge transfer molecular nanosheet described in this work, it could serve as direct convertors between light and mechanical energy and can play a role in a vast array of technological applications, including light-controlled gas storage, microactuation, microsensing, wireless photoactuators, adaptive optics, and artificial muscle technologies, etc.

Next, the researchers will actively investigate other organic charge transfer systems to elucidate if they display similar photostrictive properties. Secondly, they are planning to investigate the potential flexible photostrictive applications of charge transfer DBTTF/C60 molecular nanosheet, like wearable photoactuators.

"According to the mechanism presented in this work, we propose that there should be some other kinds of less-complicated charge transfer materials, especially in the form of two dimension, possessing high photostrictive effect and fast response time," Ren concludes. "In addition, for next-generation of flexible electronics, the flexibility and stretchability are also need to be taken into consideration, while this is challenge for conventional photostrictive materials."